Transfer processes involve physically transferring an image from one substrate to another and can be achieved in several ways. One method is melt transfer printing where a design is first printed on paper using a waxy ink. Melt transfer printing has been used since the nineteenth century to transfer embroidery designs to fabric. A design is printed on paper using a waxy ink, then transferred with heat and pressure to a final substrate. The Star process, developed by Star Stampa Artistici di Milano, uses a paper that is coated with waxes and dispersing agents. The design is printed onto the coated paper by a gravure printing process using an oil and wax based ink. The print is then transferred to fabric by pressing the composite between heated calendar rollers at high pressure. The ink melts onto the final substrate carrying the coloring materials with it. Fabrics printed in such a method using direct dyes are then nip-padded with a salt solution and steamed. Vat dyes can also be used in the ink, but the fabric must be impregnated with sodium hydroxide and hydros solution and steamed. The residual waxes from the transfer ink are removed during washing of the fabric.
Conventional heat-melt thermal printing uses primarily non-active wax materials such as hydrocarbon wax, carnauba wax, ester wax, paraffin wax, etc. as heat-melt material. Though these wax or wax-like materials serve the purpose of heat-melt very well, they present problems when the product is used in a further transfer process, especially when the image is transferred to a fibrous material, such as a textile. The conventional wax materials are not chemically bonded or otherwise permanently bonded to the substrate, but are temporarily and loosely bound to the final substrate by the melting of wax during the transfer process. The resulting image is not durable, with the wax materials being washed away during laundering of textile substrates on which the image is transferred, particularly if hot water is used, along with the dyes or colorants which form the image in the thermal ink layer. Since, in most cases, the ink layer composition has a major percentage of wax or wax-like material, and the colorants used in such composition are either wax soluble and/or completely dispersed in wax material, the associated problems of poor wash fastness, color fastness, and poor thermal stability, of the final product result in rapid and severe image quality deterioration during the usage of the product.
Another method of transfer printing is film release transfer. Here the image is printed onto a paper substrate coated with a film of heat tackifiable resin. Upon application of heat and pressure to the back side of the image, the entire film containing the image is transferred to the final substrate. A process of thermal transfer wherein the ink physically bonds to the substrate is described in Hare, U.S. Pat. No. 4,773,953. The resulting image, as transferred, is a surface bonded image with a raised, plastic-like feel to the touch. Thermal transfer paper can transfer an image to a final substrate such as cotton, however, this method has several limitations. First, the entire sheet is transferred, not just the image. Second, such papers are heavily coated with polymeric material to bind the image onto the textile. This material makes the transfer area very stiff and has poor dimensional stability when stretched. Finally, the laundering durability is not improved to acceptable levels. The thermal transfer paper technology (cited Hare patent) only creates a temporary bond between the transfer materials and the final substrate. This bond is not durable to washing.
Another method of transfer employs the use of heat activated, or sublimation, dyes. One form of an appropriate transfer process using sublimation inks is described in Hale, et. al., U.S. Pat. No. 5,601,023, the teachings of which are incorporated herein by reference. In this situation, an image is printed onto an intermediate medium, such as paper, followed by application of heat and pressure to the back side of the intermediate medium while in contact with a final substrate. The dyes then vaporize and are preferentially absorbed by the final substrate. Sublimation dyes yield excellent results when a polyester substrate is used and are highly resistant to fading and abrasion caused by laundering. These dyes, however, have a limited affinity for substrates other than polyester, and give poor results on natural fibers such as cotton and wool.
A method of preparing an image receiving sheet for sublimation transfer utilizing isocyanate groups is described in DeVries, U.S. Pat. No. 4,058,644. Here, a polyisocyanate is reacted with a polyol to form a polyurethane containing free or blocked isocyanate groups. A print paste containing this polymer along with a sublimation dye is coated onto a paper substrate via roller coating, brush coating, silk screening, or similar method. The image may then be heat transferred to a cotton substrate. The application of heat to the back side of the printed image activates the sublimation dye as well as the blocked isocyanate groups. The isocyanate groups become unblocked at the transfer temperature and available to react with hydroxyl groups on the cellulose fibers, therefore forming a chemical bond with the cellulose fiber.
DeFago, et. al. in U.S. Pat. Nos. 3,940,246 and 4,029,467 also take advantage of the reactivity of isocyanate groups. Here, sublimation dyes containing active hydrogen may be combined in a print paste with a free or blocked isocyanate. The print paste is coated on a carrier sheet by a process such as silk screen, planographic, or relief-printing, then heat transferred to a textile substrate. The isocyanate groups may react with the active hydrogen on the sublimation dye and/or with an active hydrogen on a final substrate.
Yoshimura in U.S. Pat. No. 5,432,258 describes the use of a thermosetting adhesive layer coated onto a printed image, then heat transferred onto a ceramic substrate. The thermosetting adhesive layer contains an alkyl (meth)acrylate polymer and/or α,β-unsaturated carboxylic acid and a cross-linking agent, such as an isocyanate. Upon heat transfer, the isocyanate reacts with the hydroxyl and carboxyl groups of the alkyl (meth)acrylate and α-β-unsaturated carboxylic acid to form a resin that enhances adhesion of the image to the ceramic substrate.